Add backslash to BOOT_ARG otherwise cloud-init is not working.

Hi,

We need to add backslash between "ds=nocloud-net" and ";s=${METADATA_URL}" otherwise $METADATA_URL is not populated in /proc/cmdline and cloud-init is not working.

For exemple, without:
```BOOT_IMAGE=/vmlinuz-3.12-0.bpo.1-amd64 root=/dev/mapper/rootfs-slash ro console=tty0 console=ttyS1,115200n8 ds=nocloud-net```

And with backslash:
```BOOT_IMAGE=/vmlinuz-3.12-0.bpo.1-amd64 root=/dev/mapper/rootfs-slash ro console=tty0 console=ttyS1,115200n8 ds=nocloud-net;s=http://10.153.19.10/```

In my previous use it was working, I don't know when this bug has appeared.

Thanks in advance.